Enzymatic activation of oleuropein: A protein crosslinker used as a chemical defense in the privet tree

Abstract

Leaves of the privet tree, Ligustrum obtusifolium, contain a large amount of oleuropein, a phenolic secoiridoid glycoside, which is stably kept in a compartment separate from activating enzymes. When the leaf tissue is destroyed by herbivores, enzymes localized in organelles start to activate oleuropein into a very strong protein denaturant that has protein-crosslinking and lysine-decreasing activities. These activities are stronger than ever reported from plant systems and have adverse effects against herbivores by decreasing the nutritive value of dietary protein completely. We report here that strong oleuropein-specific ␤-glucosidase in organelles activates oleuropein by converting the secoiridoid glucoside moiety of oleuropein into a glutaraldehyde-like structure, which is also an ␣,␤-unsaturated aldehyde. Oleuropein activated by ␤-glucosidase had very strong proteindenaturing, protein-crosslinking, and lysine-alkylating activities that are very similar to, but stronger than, those of glutaraldehyde. Aucubin, another iridoid glycoside, had similar activities after ␤-glucosidase treatment. We also detected polyphenol oxidase activity in organelles that activate the dihydroxyphenolic moiety to have protein-crosslinking activities. These data suggest that the privet tree has developed an effective defense mechanism with oleuropein, a unique multivalent alkylator ideal as a protein-crosslinker. Our results that iridoid glycosides are precursors of alkylators may elucidate the chemical bases that underlie various bioactivities and ecological roles of iridoid glycosides.

Oleuropein aglycone protects against pyroglutamylated-3 amyloid-ß toxicity: biochemical, epigenetic and functional correlates

“…Oleuropein (Extrasynthase, Genay Cedex, France) deglycosylation was performed according to Konno et al (1999) with minor modifications. Briefly, a 10 mM solution of glycated oleuropein in 0.1 M sodium phosphate buffer, pH 7.0, was incubated with 6.0 I.U./mL of bglycosidase overnight at room temperature in the dark.…”

Chemical Mediation of Interactions Among Marine Organisms

“…Examples include plants that produce HCN from organic precursors, 158 ' 159 plants that convert glucosinolates to thiocyanates and isothiocyanates 160 ' 161 after tissue damage and the activation of oleuropein, 5 a phenolic secoiridoid glycoside with strong protein denaturing activity. 162 In these cases, precursor compounds are compartmentalized and physically separated from the enzymes that activate them; however, we know little about the mechanisms involved in activation in marine plants.…”

Deciphering the route to cyclic monoterpenes in Chrysomelina leaf beetles: source of new biocatalysts for industrial application?

“…To date, it has been shown that iridoid toxicity can be attributed to the highly reactive aglycones that are released from the corresponding nontoxic iridoid glucosides that are often safely stored in plant organelles [18,19]. Glycoside hydrolysis can be achieved nonenzymatically or enzymatically by β-glucosidases (hydrolases, EC 3.2.1.21) produced by the plants themselves [20][21][22] or by their enemies [12,13,23,24].…”